5,116 research outputs found
A Revisit to Top Quark Forward-Backward Asymmetry
We analyze various models for the top quark forward-backward asymmetry
() at the Tevatron, using the latest CDF measurements on different
s and the total cross section. The axigluon model in Ref. \cite{paul}
has difficulties in explaining the large rapidity dependent asymmetry and mass
dependent asymmetry simultaneously and the parameter space relevant to
is ruled out by the latest dijet search at ATLAS. In contrast to
Ref. \cite{cp}, we demonstrate that the large parameter space in this model
with a flavor symemtry is not ruled out by flavor physics. The
-channel flavor-violating \cite{hitoshi},
\cite{waiyee} and diquark \cite{tim} models all have parameter
regions that satisfy different measurements within 1 .
However, the heavy model which can be marginally consistent with
the total cross section is severely constrained by the Tevatron direct search
of same-sign top quark pair. The diquark model suffers from too large total
cross section and is difficult to fit the invariant mass
distribution. The electroweak precision constraints on the model based on
- mixings is estimated and the result is rather weak (
GeV). Therefore, the heavy model seems to give the best fit for
all the measurements. The model predicts the signal
from production and is 10%-50% of SM at the 7 TeV LHC.
Such resonance can serve as the direct test of the model.Comment: 25 pages, 7 figures, 1 tabl
Neutrino Constraints on Inelastic Dark Matter after CDMS II
We discuss the neutrino constraints from solar and terrestrial dark matter
(DM) annihilations in the inelastic dark matter (iDM) scenario after the recent
CDMS II results. To reconcile the DAMA/LIBRA data with constraints from all
other direct experiments, the iDM needs to be light ( GeV) and
have a large DM-nucleon cross section ( 10 pb in the
spin-independent (SI) scattering and 10 pb in the
spin-dependent (SD) scattering). The dominant contribution to the iDM capture
in the Sun is from scattering off Fe/Al in the SI/SD case. Current bounds from
Super-Kamiokande exclude the hard DM annihilation channels, such as ,
, and . For soft channels such as and
, the limits are loose, but could be tested or further constrained
by future IceCube plus DeepCore. For neutrino constraints from the DM
annihilation in the Earth, due to the weaker gravitational effect of the Earth
and inelastic capture condition, the constraint exists only for small mass
splitting 40 keV and GeV even in the channel.Comment: 11 pages, 8 figure
Fate of non-Fermi liquid behavior in QED at finite chemical potential
The damping rate of two-dimensional massless Dirac fermions exhibit non-Fermi
liquid behavior, , due to gauge field at zero
temperature and zero chemical potential. We study the fate of this behavior at
finite chemical potential. We fist calculate explicitly the temporal and
spatial components of vacuum polarization functions. The analytical expressions
imply that the temporal component of gauge field develops a static screening
length at finite chemical potential while the transverse component remains
long-ranged owing to gauge invariance. We then calculate the fermion damping
rate and show that the temporal gauge field leads to normal Fermi liquid
behavior but the transverse gauge field leads to non-Fermi liquid behavior
at zero temperature. This energy-dependence is more
regular than and does not change as chemical potential
varies.Comment: 12 pages, 1 figur
Dynamic Cycling of t-SNARE Acylation Regulates Platelet Exocytosis
Platelets regulate vascular integrity by secreting a host of molecules that promote hemostasis and its sequelae. Given the importance of platelet exocytosis, it is critical to understand how it is controlled. The t-SNAREs, SNAP-23 and syntaxin-11, lack classical transmembrane domains (TMDs), yet both are associated with platelet membranes and redistributed into cholesterol-dependent lipid rafts when platelets are activated. Using metabolic labeling and hydroxylamine (HA)/HCl treatment, we showed that both contain thioester-linked acyl groups. Mass spectrometry mapping further showed that syntaxin-11 was modified on cysteine 275, 279, 280, 282, 283, and 285, and SNAP-23 was modified on cysteine 79, 80, 83, 85, and 87. Interestingly, metabolic labeling studies showed incorporation of [3H]palmitate into the t-SNAREs increased although the protein levels were unchanged, suggesting that acylation turns over on the two t-SNAREs in resting platelets. Exogenously added fatty acids did compete with [3H]palmitate for t-SNARE labeling. To determine the effects of acylation, we measured aggregation, ADP/ATP release, as well as P-selectin exposure in platelets treated with the acyltransferase inhibitor cerulenin or the thioesterase inhibitor palmostatin B. We found that cerulenin pretreatment inhibited t-SNARE acylation and platelet function in a dose- and time-dependent manner whereas palmostatin B had no detectable effect. Interestingly, pretreatment with palmostatin B blocked the inhibitory effects of cerulenin, suggesting that maintaining the acylation state is important for platelet function. Thus, our work shows that t-SNARE acylation is actively cycling in platelets and suggests that the enzymes regulating protein acylation could be potential targets to control platelet exocytosis in vivo
Environmental Effect on the Associations of Background Quasars with Foreground Objects: II. Numerical Simulations
Using numerical simulations of cluster formation in the standard CDM model
(SCDM) and in a low-density, flat CDM model with a cosmological constant
(LCDM), we investigate the gravitational lensing explanation for the reported
associations between background quasars and foreground clusters. Under the
thin-lens approximation and the unaffected background hypothesis , we show that
the recently detected quasar overdensity around clusters of galaxies on scales
of arcminutes cannot be interpreted as a result of the gravitational
lensing by cluster matter and/or by their environmental and projected matter
along the line of sight, which is consistent with the analytical result based
on the observed cluster and galaxy correlations (Wu, et al. 1996). It appears
very unlikely that uncertainties in the modeling of the gravitational lensing
can account for the disagreement between the theoretical predictions and the
observations. We conclude that either the detected signal of the quasar-cluster
associations is a statistical fluke or the associations are are generated by
mechanisms other than the magnification bias.Comment: 15 pages, 5 figures, accepted for publication in Ap
The Dependence of the Occupation of Galaxies on the Halo Formation Time
We study the dependence of the galaxy contents within halos on the halo
formation time using two galaxy formation models, one being a semianalytic
model utilizing the halo assembly history from a high resolution N-body
simulation and the other being a smoothed particle hydrodynamics simulation
including radiative cooling, star formation, and energy feedback from galactic
winds. We confirm the finding by Gao et al. that at fixed mass, the clustering
of halos depends on the halo formation time, especially for low-mass halos.
This age dependence of halo clustering makes it desirable to study the
correlation between the occupation of galaxies within halos and the halo age.
We find that, in halos of fixed mass, the number of satellite galaxies has a
strong dependence on halo age, with fewer satellites in older halos. The
youngest one-third of the halos can have an order of magnitude more satellites
than the oldest one-third. For central galaxies, in halos that form earlier,
they tend to have more stars and thus appear to be more luminous, and the
dependence of their luminosity on halo age is not as strong as that of stellar
mass. The results can be understood through the star formation history in halos
and the merging of satellites onto central galaxies. The age dependence of the
galaxy contents within halos would constitute an important ingredient in a more
accurate halo-based model of galaxy clustering.Comment: 4 pages, 2 figures, Accepted by ApJ Letters, emulateapj layout. Minor
changes. Poisson errors added in Figure 1. We remove the last figure, which
is available on http://bias.cosmo.fas.nyu.edu/galevolution/hod/f3.ep
Optical effects of spin currents in semiconductors
A spin current has novel linear and second-order nonlinear optical effects
due to its symmetry properties. With the symmetry analysis and the eight-band
microscopic calculation we have systematically investigated the interaction
between a spin current and a polarized light beam (or the "photon spin
current") in direct-gap semiconductors. This interaction is rooted in the
intrinsic spin-orbit coupling in valence bands and does not rely on the Rashba
or Dresselhaus effect. The light-spin current interaction results in an optical
birefringence effect of the spin current. The symmetry analysis indicates that
in a semiconductor with inversion symmetry, the linear birefringence effect
vanishes and only the circular birefringence effect exists. The circular
birefringence effect is similar to the Faraday rotation in magneto-optics but
involves no net magnetization nor breaking the time-reversal symmetry.
Moreover, a spin current can induce the second-order nonlinear optical
processes due to the inversion-symmetry breaking. These findings form a basis
of measuring a pure spin current where and when it flows with the standard
optical spectroscopy, which may provide a toolbox to explore a wealth of
physics connecting the spintronics and photonics.Comment: 16 pages, 7 fig
Dose-related liver injury of Geniposide associated with the alteration in bile acid synthesis and transportation.
Fructus Gardenia (FG), containing the major active constituent Geniposide, is widely used in China for medicinal purposes. Currently, clinical reports of FG toxicity have not been published, however, animal studies have shown FG or Geniposide can cause hepatotoxicity in rats. We investigated Geniposide-induced hepatic injury in male Sprague-Dawley rats after 3-day intragastric administration of 100 mg/kg or 300 mg/kg Geniposide. Changes in hepatic histomorphology, serum liver enzyme, serum and hepatic bile acid profiles, and hepatic bile acid synthesis and transportation gene expression were measured. The 300 mg/kg Geniposide caused liver injury evidenced by pathological changes and increases in serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) and γ-glutamytransferase (γ-GT). While liver, but not sera, total bile acids (TBAs) were increased 75% by this dose, dominated by increases in taurine-conjugated bile acids (t-CBAs). The 300 mg/kg Geniposide also down-regulated expression of Farnesoid X receptor (FXR), small heterodimer partner (SHP) and bile salt export pump (BSEP). In conclusion, 300 mg/kg Geniposide can induce liver injury with associated changes in bile acid regulating genes, leading to an accumulation of taurine conjugates in the rat liver. Taurocholic acid (TCA), taurochenodeoxycholic acid (TCDCA) as well as tauro-α-muricholic acid (T-α-MCA) are potential markers for Geniposide-induced hepatic damage
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